The microbial quality of livestock drinking water was evaluated in 473 cattle water troughs located at 99 different cattle operations. The mean log10-transformed coliform and Escherichia coli concentrations per milliliter of trough water were 1.76 +/- 1.25 (SD) and 0.98 +/- 1.06 (SD), respectively. The degree of E. coli contamination was positively associated with the proximity of the water trough to the feedbunk, protection of the trough from direct sunlight, lower concentrations of protozoa in the water, and warmer weather. Salmonella sp. were isolated from 2/235 (0.8%) troughs and shigatoxigenic-E. coli O157 was recovered from 6/473 (1.3%) troughs. Four experimental microcosms simulating cattle water troughs were used to further evaluate the effects of protozoal populations on the survival of E. coli O157 in cattle water troughs. Escherichia coli O157 of bovine fecal origin proliferated in all microcosms. Reduction of protozoal populations by treatment with cycloheximide was associated with increased persistence of E. coli O157 concentrations in the microcosms. Water troughs are a major source of exposure of cattle to enteric bacteria, including a number of foodborne pathogens, and this degree of bacterial contamination appeared to be associated with potentially controllable factors.